Fluororesins such as polytetrafluoroethylene (hereinafter referred to also as “PTFE”), a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer (hereinafter referred to also as “PFA”), an ethylene/tetrafluoroethylene copolymer (hereinafter referred to also as “ETFE”), polyvinylidene fluoride (hereinafter referred to also as “PVdF”, etc. are excellent in heat resistance, flame retardancy, chemical resistance, weather resistance, non-stickiness, low friction property, low dielectric property, etc. and thus have been used in a wide range of fields such as coating materials for heat-resistant flame retardant electric wires, chemical plant corrosion-resistant piping materials, agricultural vinyl house materials, release coat materials for kitchen utensils, etc.
Among such fluorine resins, a perfluoro type fluororesin such as PTFE or PFA is excellent in chemical resistance and heat resistance, as compared with a partially fluorinated type fluororesin such as ETFE or PVdF. But, on the other hand, since its intermolecular force is low, the cohesiveness is small, and its elastic modulus or abrasion resistance tends to be low. A material having a low elastic modulus or abrasion resistance has such a problem that it tends to be easily deformed particularly in an application as a slide member or electric wire covering material to be used under a high load or at a high temperature, or it tends to be easily scraped by abrasion with the other material.
In order to improve the elastic modulus or abrasion resistance of a fluororesin, attempts have been made to crosslink the fluororesin. For example, Patent Document 1 has proposed a method wherein ETFE kneaded with a cross-linking aid is extrusion-molded under a predetermined condition, and the obtained molded product is irradiated with an electron beam and cross-linked.
This method is effective for a partially fluorinated type fluororesin, but when it is applied to a perfluoro type fluororesin, the molecular main chain is cut by electron beam irradiation, and embrittlement is promoted, thus leading to such a problem that physical properties such as mechanical strength, etc. tend to be deteriorated.
Against this problem, attempts have been made to crosslink a perfluoro type fluororesin.
In Patent Document 2, a perfluoro type fluororesin is irradiated with ionizing radiation in the absence of oxygen at a temperature of at least the crystalline melting temperature and cross-linked, in an attempt to improve abrasion resistance, wear resistance, load resistance, etc.
In Patent Document 3, it has been attempted to improve mechanical properties by incorporating a fluorinated divinyl polyether compound as a cross-linking agent to PFA having a heat of crystal fusion of not more than 10 J/g, followed by crosslinking by irradiation with radiation.
However, in the method of Patent Document 2, it is necessary to irradiate the electron beam at a temperature of at least the crystalline melting point of the fluororesin, whereby when an electron beam is irradiated to the molded article, if the melt viscosity of the fluororesin is low, the shape of the molded article cannot be maintained. For this reason, the fluororesin that can actually be used is limited to PTFE having a high melt viscosity. In addition, also in the case of PTFE, there will be such a problem as a dimensional change or compression bonding with other material at the time of melting. On the other hand, if at the stage of pellets before molding, the fluororesin is cross-liked by irradiation of an electron beam, the fluidity decreases and the moldability tends to be deteriorated.
In the method of Patent Document 3, it is said to be possible to conduct crosslinking at a temperature of not higher than 100° C. However, in this method, it is necessary to use a cross-linking agent containing a vinyl group, whereby after the crosslinking, the heat resistance and heat stability tend to be deteriorated by vinyl groups remaining as unreacted. Therefore, the use of a crosslinking agent containing a vinyl group is not preferred for applications such as slide members or electric wire covering materials used at high temperatures. Also in Patent Document 3, irradiation of the electron beam is conducted in the absence of oxygen.
Thus, a technique has not yet been established whereby it is possible to improve the abrasion resistance of a perfluoro type fluororesin, by electron beam irradiation in air at a temperature where the fluororesin does not melt, without combination with a crosslinking agent.